Sensitive bidirectional static switch

ABSTRACT

A monolithic bidirectional switch formed in a semiconductor substrate of type N, including a first main vertical thyristor, the rear surface layer of which is of type P, a second main vertical thyristor, the rear surface layer of which is of type N, an auxiliary vertical thyristor, the rear surface layer of which is of type P and is common with that of the first main thyristor, a peripheral region of type P especially connecting the rear surface layer of the auxiliary thyristor to the layer of this thyristor located on the other side of the substrate, a first metallization on the rear surface side, a second metallization on the front surface side connecting the front surface layers of the first and second thyristors. An additional region has a function of isolating the rear surface of the auxiliary thyristor and the first metallization.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to making, in monolithic form,bidirectional switches of medium power.

[0003] 2. Discussion of the Related Art

[0004] The most current static bidirectional switches are triacs. Atriac corresponds to the antiparallel association of two thyristors. Itcan thus be directly connected in an A.C. network, for example, themains. The gate of a conventional triac corresponds to the cathode gateof one at least of the two thyristors forming it and is referenced tothe electrode located on the front surface of this triac, that is, thesurface that includes the gate terminal, while the other triac surfaceis typically connected to a heat sink and to the ground.

[0005] Bidirectional switches of the type described in European patentapplication No. 0817277, the triggering of which is ensured by applyinga voltage between a control electrode located on the front surface ofthe component and a main electrode located on the opposite surface ofthe component, will more specifically be considered hereafter.

[0006]FIG. 1 shows an equivalent electric diagram of such abidirectional switch. A control electrode G of the bidirectional switchis connected to the emitter of a bipolar transistor T, the collector ofwhich is connected the anode gates of first and second thyristors Th1and Th2 placed in antiparallel between two terminals A1 and A2. TerminalA1 corresponds to the anode of thyristor Th1 and to the cathode ofthyristor Th2. Terminal A1 is also connected to the base of transistorT. Terminal A2 corresponds to the anode of thyristor Th2 and to thecathode of thyristor Th1.

[0007]FIG. 2 is a simplified cross-section view of an example ofmonolithic embodiment of the bidirectional switch described in relationwith FIG. 1. Transistor T is formed in the left-hand portion of thedrawing, thyristor Th1 at the center, and thyristor Th2 to the rightthereof.

[0008] The structure of FIG. 2 is formed from an N-type lightly-dopedsemiconductor substrate 1. The anode of thyristor Th1 corresponds to aP-type layer 2 that is formed on the rear surface side of substrate 1.Its cathode corresponds to an N-type region 3 formed on the frontsurface side in a P-type well 4. The anode of thyristor Th2 correspondsto a P-type well 5 formed on the front surface side and its cathodecorresponds to an N-type region 6 formed on the rear surface side inlayer 2. The periphery of the structure is formed of a heavily-dopedP-type layer 7 extending from the front surface to P-type layer 2.Conventionally, region 7 is obtained by drive-in from the two substratesurfaces. The rear surface is coated with a metallization MIcorresponding to first terminal A1 of the bidirectional switch. Theupper surfaces of regions 3 and 5 are coated with a second metallizationM2 corresponding to second terminal A2 of the bidirectional switch. AnN-type region 8 is formed, on the front surface side, in a P-type well 9in contact with peripheral region 7. The surface of region 8 iscontacted by a metallization M3 connected to control terminal G of thebidirectional switch. A metallization M4 may be formed on the uppersurface of peripheral region 7. Metallization M4 is not connected to anexternal terminal. As an alternative, well 9 may be separated fromperipheral region 7 and electrically connected thereto via metallizationM4.

[0009] The operation of this bidirectional switch is the following.

[0010] When terminal A2 is negative with respect to terminal A1,thyristor Th1 is likely to be on. If a sufficiently negative voltagewith respect to metallization M1 is applied to gate G, the base-emitterjunction of transistor T is forward biased and this transistor turns on.A vertical current i_(c) shown in dotted lines in FIG. 2 thus flows frommetallization M1, through the forward junction between layer 2 andsubstrate 1, then into regions 1, 9 and 8 corresponding to transistor T.Carriers are thus generated at the level of the junction betweensubstrate 1 and well 9 near the junction between substrate 1 and well 4,and thyristor Th1 is turned on. It can also be considered that anauxiliary vertical NPNP thyristor including regions 8-9-1-2, region 9 ofwhich forms the cathode gate region, has been triggered.

[0011] Similarly, when terminal A2 is positive with respect to terminalA1, applying a negative voltage on terminal G turns transistor T on. Thecarriers present in the vicinity of the junction between substrate 1 andlayer 2 turn thyristor Th2 on, as will be better understood by referringto the simplified top view of FIG. 4 in which it can be seen that theregion corresponding to transistor T is a neighbor to a portion of eachof thyristors Th1 and Th2.

[0012] Practice reveals that this type of bidirectional switch has anon-optimal control responsiveness, that is, especially, that thecurrent required to trigger thyristor Th1 is of several hundreds ofmilliamperes.

SUMMARY OF THE INVENTION

[0013] An object of the present invention is to provide a novelembodiment in monolithic form of a bidirectional switch of the abovementioned type that exhibits a greater control responsiveness ofthyristor Th1.

[0014] To achieve this and other objects, the present invention providesa monolithic bidirectional switch formed in a semiconductor substrate ofa first conductivity type having a front surface and a rear surface,including a first main vertical thyristor, the rear surface layer ofwhich is of the second conductivity type, a second main verticalthyristor, the rear surface layer of which is of the first conductivitytype, an auxiliary vertical thyristor, the rear surface layer of whichis of the second conductivity type and is common with the rear surfacelayer of the first main thyristor, a peripheral region of the secondconductivity type especially connecting the rear surface layer of theauxiliary thyristor to the layer of this thyristor located on the otherside of the substrate, a first metallization on the rear surface side, asecond metallization on the front surface side connecting the frontsurface layers of the first and second thyristors. An additional regionisolates the rear surface of the auxiliary thyristor and the firstmetallization.

[0015] According to an embodiment of the present invention, theadditional region is made of a semiconductor material of the firstconductivity type.

[0016] According to an embodiment of the present invention, thethickness of the additional region is smaller than that of the rearsurface region of the second main vertical thyristor.

[0017] According to an embodiment of the present invention, theadditional region is made of silicon oxide.

[0018] The foregoing objects, features and advantages of the presentinvention, will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic electric diagram of a conventionalbidirectional switch;

[0020]FIG. 2 is a simplified cross-section view of a conventionalembodiment of the bidirectional switch of FIG. 1;

[0021]FIG. 3 shows a simplified cross-section view of an embodimentaccording to the present invention of the bidirectional switch of FIG.1; and

[0022]FIG. 4 shows an example of a top view of the bidirectional switchof FIG. 3.

DETAILED DESCRIPTION

[0023] For clarity, the same elements have been designated by samereferences in the different drawings. Further, as usual in therepresentation of integrated circuits, FIGS. 2, 3, and 4 are not drawnto scale.

[0024]FIG. 3 is a simplified cross-section view of an embodiment of amonolithic bidirectional switch according to the present invention. Thestructure of the various areas formed in semiconductor substrate 1 isidentical to that illustrated in FIG. 2. The difference between the twodrawings is that a region 10 having an isolation function, substantiallyunder the above-mentioned auxiliary vertical thyristor, is provided onthe rear surface side, between layer 2 and metallization M1. This alsoappears from FIG. 4 in which the contour of region 10 is designated by adotted line on the bottom left-hand portion of the drawing. Layer 6, notshown in FIG. 4, occupies the entire lower surface except for the arealocated under P-type well 4 and the surface occupied by region 10.

[0025] According to an embodiment of the present invention, region 10 isformed of a semiconductor N-type doped material.

[0026] The operation of the bidirectional switch remains substantiallysimilar to what has been described in relation with FIG. 2. However,base current i_(b) of transistor T, running from metallization M1 toregion 8, is now changed by the presence of region 10, and flowsaccording to path i_(b) of FIG. 3.

[0027] The main current of the auxiliary vertical thyristor is alsochanged, as shown by arrows i_(c). It can be seen that by modifying thedimensions of region 10, the passing of current i_(c) is favored in thevicinity of the areas where it is most efficient to turn on thyristorTh1, that is, close to the limit of well 4.

[0028] Tests performed by the applicants have shown that the triggeringcurrent of thyristor Th1 is reduced or minimized when region 10 extendsto face P-type well 4 in which N-type region 3 forming the cathode ofthyristor Th1 is formed.

[0029] According to the present invention, the thickness of region 10must be sufficiently small to initially enable the starting oftransistor T by the conduction of current i_(b) from layer 2 to region 8via-peripheral region 7. Indeed, if region 10 is too thick, theremaining thickness of layer 2 between region 10 and substrate 1 resultsin the existence of too high a resistance that opposes to the flowing ofbase current i_(b).

[0030] In practice, the thickness of region 10 will be smaller than thatof layer 6. Indeed, layer 6 forms the cathode of thyristor. Th2 and itsthickness is determined by the characteristics, especially relating tothe triggering current, of this sole thyristor. The thickness of layer 6will for example be on the order of 10 to 15 μm, while the thickness ofregion 10 will be as small as possible.

[0031] According to an alternative embodiment of the present invention,region 10 is made of an insulating material, preferably silicon oxide(SiO₂).

[0032] Of course, the present invention is likely to have variousalterations, modifications, and improvements which will readily occur tothose skilled in the art. In particular, the present invention appliesto any type of monolithic realization of the bidirectional switchstructure or of a network of bidirectional switches similar to thatshown in FIG. 1. Further, all conductivity types could be inverted, withthe biasings being then correspondingly modified.

[0033] Such alterations, modifications, and improvements are intended tobe part of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A monolithic bidirectional switch formed in asemiconductor substrate of a first conductivity type having a frontsurface and a rear surface, including: a first main vertical thyristor,a rear surface layer of which is of a second conductivity type, a secondmain vertical thyristor, a rear surface layer of which is of the firstconductivity type, an auxiliary vertical thyristor, a rear surface layerof which is of the second conductivity type and is common with the rearsurface layer of the first main thyristor, a peripheral region of thesecond conductivity type connecting the rear surface layer of theauxiliary thyristor to the layer of this thyristor located on the otherside of the substrate, a first metallization on the rear surface side, asecond metallization on the front surface side connecting the frontsurface layers of the first and second thyristors, and an additionalregion that isolates the rear surface of the auxiliary thyristor and thefirst metallization.
 2. The bidirectional switch of claim 1, wherein theadditional region is made of a semiconductor material of the firstconductivity type.
 3. The bidirectional switch of claim 2, wherein thethickness of the additional region is smaller than that of the rearsurface region of the second main vertical thyristor.
 4. Thebidirectional switch of claim 1, wherein the additional region is madeof silicon oxide.